Experiments will be done to study the immune status in various models of specific unresponsiveness and to try to elucidate the mechanisms underlying the unresponsiveness. Both thymectomized and non-thymectomized mice treated with ALS and injected with low doses of donor bone marrow are unresponsive to donor antigens as shown by prolonged skin allograft survival. This unresponsiveness is a type of active enhancement since mice bearing perfect skin grafts exhibit cell-mediated immunity (CMI) against donor antigens and also have blocking factors present in their serum (SBF). These mice are not chimeras. In contrast, thymectomized ALS treated mice injected with large doses of hybrid lymph node-spleen cells are chimeras. They have no evidence of CMI against antigens. Experiments will compare these models with reference to (1) response to Cytoxan treatment before antigen is given (2) ability of host lymphocytes to form allorosettes with donor erythrocytes and (3) presence of suppressor cells in lymphoid organs and ability of these cells to transfer suppression to secondary syngeneic hosts. Other experiments will investigate the ability of platelets to induce unresponsiveness in ALS treated mice.